Describing Motion
3
Motion
Chapter3,Section2:Acceleration
Acceleration
3-2
Acceleration, Speed and Velocity
• Acceleration is the rate of change of
velocity. When the velocity of an object
changes, the object is accelerating.
• A change in velocity can be either a change
in how fast something is moving, or a change
in the direction it is moving.
• Acceleration occurs when an object changes
its speed, its direction, or both.
Acceleration
3-2
Speeding Up and Slowing Down
• When you think of acceleration, you
probably think of something speeding up.
However, an object that is slowing down also
is accelerating.
• Acceleration also has direction, just as
velocity does.
Acceleration
3-2
Speeding Up and Slowing Down
• If the acceleration is in the same direction as the
velocity,
the speed
increases and
the acceleration
is positive.
Acceleration
3-2
Speeding Up and Slowing Down
• If the speed decreases, the acceleration is in
the opposite
direction from
the velocity,
and the
acceleration is
negative.
Acceleration
3-2
Changing Direction
• A change in velocity can be either a change
in how fast something is moving or a change
in the direction of movement.
• Any time a moving object changes direction,
its velocity changes and it is accelerating.
Acceleration
3-2
Changing Direction
• The speed of the
horses in this
carousel is
constant, but the
horses are
accelerating
because their
direction is
changing
constantly.
Acceleration
3-2
Calculating Acceleration
• To calculate the acceleration of an object, the
change in velocity is divided by the length of
time over which the change occurred.
Acceleration
3-2
Calculating Positive Acceleration
• How is the acceleration for an object that is
speeding up different from that of an object
that is slowing down?
• Suppose a jet airliner starts at rest at the end
of a runway and reaches a speed of 80 m/s in
20 s.
Acceleration
3-2
Calculating Positive Acceleration
• Its acceleration can be calculated as follows:
Acceleration
3-2
Calculating Positive Acceleration
• The airliner is
speeding up, so the
final speed is
greater than the
initial speed and
the acceleration is
positive.
Acceleration
3-2
Calculating Negative Acceleration
• Now imagine that a skateboarder is moving
in a straight line at a constant speed of 3 m/s
and comes to a
stop in 2 s.
• The final speed
is zero and the
initial speed
was 3 m/s.
m/s2
m/s2
Acceleration
3-2
Calculating Negative Acceleration
• The skateboarder is slowing down, so the
final speed is less than the initial speed and
the acceleration is
negative.
• If moving in the
positive direction, he
acceleration will be
positive if an object
is speeding up and
negative if the object
is slowing down.
Section Check
3-2
Question 1
Acceleration is the rate of change of
__________.
Section Check
3-2
Answer
The correct answer is velocity. Acceleration
occurs when an object changes its speed,
direction, or both.
Section Check
3-2
Question 2
Which is NOT a form of acceleration?
A.
B.
C.
D.
maintaining a constant speed and direction
speeding up
slowing down
turning
Section Check
3-2
Answer
The answer is A. Any change of speed or
direction results in acceleration.
Section Check
3-2
Question 3
What is the acceleration of a hockey player
who is skating at 10 m/s and comes to a
complete stop in 2 s?
A.
B.
C.
D.
5 m/s2
-5 m/s2
20 m/s2
-20 m/s2
Section Check
3-2
Answer
The answer is B. Calculate acceleration by
subtracting initial velocity (10 m/s) from
final velocity (0), then dividing by the time
interval (2s).
(0 m/s – 10 m/s) = – 5 m/s
2s